Chromatin is the complex combination of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells and is divided between heterochromatin (condensed) and euchromatin (extended) forms. A range of different states of condensation are possible and the tightness of this structure varies during the cell cycle, being the most compact during the process of cell division. The major components of chromatin are DNA and proteins. Histones are the chief protein components of chromatin, acting as spools around which DNA winds. The basic building blocks of chromatin are nucleosomes, each of which is composed of 146 base pairs of DNA wrapped around a histone octamer that consists of 2 copies of each H2A, H2B, H3 and H4. The functions of chromatin are to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis, and to serve as a mechanism to control expression and DNA replication. Chromatin contains genetic material serving as instructions to direct cell functions. The genomes of eukaryotic organisms are highly organised within the nucleus of the cell. The chromatin structure is controlled by a series of post translational modifications to histone proteins, notably histones H3 and H4, and most commonly within the “histone tails” which extend beyond the core nuclerosome structure. Histone tails tend to be free for protein—protein interaction and are also the portion of the histone most prone to post-translational modification. These modifications include acetylation, methylation, phosphorylation, ubiquitinylation, SUMOylation. These epigenetic marks are written and erased by specific enzymes, which place the tags on specific residues within the histone tail, thereby forming an epigenetic code, which is then interpreted by the cell to allow gene specific regulation of chromatin structure and thereby transcription.
Of all classes of proteins, histones are amongst the most susceptible to post-translational modification. Histone modifications are dynamic, as they can be added or removed in response to specific stimuli, and these modifications direct both structural changes to chromatin and alterations in gene transcription. Lysine acetylation is a histone modification that forms an epigenetic mark on chromatin for bromodomain-containing proteins to dock and in turn, regulate gene expression. Distinct classes of enzymes, namely histone acetyltransferases (HATs) and histone deacetylases (HDACs), acetylate or de-acetylate specific histone lysine residues (1).
The bromodomain is currently the only protein domain known to specifically bind to acetylated lysine residues in histone tails. Bromodomains, which are approximately 110 amino acids long, are found in a large number of chromatin-associated proteins and have now been identified in approximately 70 human proteins, often adjacent to other protein motifs (2,3). Proteins that contain a bromodomain may contain additional bromodomains, as well as other functional motifs. For example, many HATs also contain a bromodomain (2). Interactions between bromodomains and modified histones may be an important mechanism underlying chromatin structural changes and gene regulation. Bromodomain containing proteins have been implicated in disease processes including cancer, inflammation and viral replication. The development of inhibitors to bromodomains is thus an attractive means for controlling gene expression, and there is a need in the art to regulate bromodomain binding to acetylated lysine in order to control gene expression.
The present inventors have identified bromodomains involved in the inflammatory response. Inhibiting these bromodomains by inhibiting expression and/or function therefore would provide a novel approach to the treatment of autoimmune and inflammatory diseases or conditions.